Plural (for example, two) kinds of molten materials are supplied into manifolds respectively provided for accepting them individually, and from the respective manifolds, the respective molten materials are made to flow through plural tiny holes or plural slits, to form layer streams of the plural molten materials. The layer streams of the plural molten materials are then laminated to form a multilayer molten material sheet, and the sheet is discharged from a slit die extending in the direction perpendicular to the direction (in the width direction of the sheet) in which the respective layers of the molten materials are laminated, to form a laminated sheet (such method is described in and known by, for example, the Patent Document 1, the Patent Document 2 or the Patent Document 3). The laminated sheet discharged from the die is used as a multilayer film as it is or after it is stretched or post-treated in any other way.
A typical example of an apparatus for producing the laminated sheet is shown in FIG. 1. In FIG. 1, the laminated sheet production apparatus is composed of a molten resin introducing pipe 1 for being supplied one molten resin A, a molten resin introducing pipe 2 for being supplied another molten resin B, a multilayer feed block 3 for forming layer streams consisting of the molten resin A supplied from the molten resin introducing pipe 1 and the molten resin B supplied from the molten resin introducing pipe 2, a conduit pipe 4 for allowing the formed layer streams to flow through it, a die 5 for adjusting the width and thicknesses of the layer streams fed through the conduit pipe 4 to predetermined values, and discharging the adjusted layer streams, to form a laminated sheet having the molten material A and the molten material B laminated alternately, and a casting drum 7 for cooling and solidifying the laminated sheet 6 discharged from the die 5. The laminated sheet solidified by the casting drum 7 is usually called an un-drawn film 8. The un-drawn film 8 is usually fed to a drawing step (not shown in the drawing) as indicated by arrow NS, and drawn in one or two directions, as a multilayer film.
The multilayer feed block 3 has in it, a manifold connected with the molten material introducing pipe 1, a manifold connected with the molten material introducing pipe 2, plural slits arranged with predetermined intervals kept between them, and a laminating portion for laminating the streams of the respective molten materials having passed through the slits. The plural slits are classified into two groups, and the plural slits of one group are open to the outlet of the manifold connected with the molten material introducing pipe 1, while the plural slits of the other group are open to the outlet of the manifold connected with the molten material introducing pipe 2. The outlet of the laminating portion communicates with the conduit pipe 4.
The basic constitution of the laminated sheet production apparatus of the invention is substantially the same as the basic constitution of the laminated sheet production apparatus shown in FIG. 1. However, the laminated sheet production apparatus of the invention is characterized by the structure of the multilayer feed block used therein.
An example of the multilayer feed block used in the conventional laminated sheet production apparatus is shown in FIG. 11. FIG. 11 shows spaces formed in the multilayer feed block.
In FIG. 11, a multilayer feed block 101 has a resin introducing path 102 installed for introducing a molten resin A into the block 101 and a resin introducing path 103 installed for introducing a molten resin B into the block. The multilayer feed block 101 is internally provided with a manifold 104 connected with the resin introducing path 102 and a manifold 105 connected with the resin introducing path 103. The manifold 104 distributes the flow of the molten resin A introduced from the resin introducing path 102 over the entire width in the longitudinal direction of the multilayer feed block 101 (X-axis direction shown in FIG. 11). The manifold 105 distributes the flow of the molten resin B introduced from the resin introducing path 103 over the entire width in the longitudinal direction of the multilayer feed block 101 (X-axis direction shown in FIG. 11).
Furthermore, in the multilayer feed block 101, numerous slits are provided with predetermined intervals 110 kept between them. The numerous slits comprise a group of slit comprising plural slits 108 and a group of slit comprising plural slits 109. The slits 108 and the slits 109 are arranged alternately with intervals 110 kept between them. The inlet of each of the slits 108 is connected with the outlet of each of tiny holes 106, and the inlets of the tiny holes 106 are connected with the manifold 104. The inlet of each of the slits 109 is connected with outlet of each of tiny holes 107, and the inlets of the tiny holes 107 are connected with the manifold 105.
Furthermore, the multilayer feed block 101 is internally provided with a laminating portion (not shown in the drawing) connected with the outlets of the respective slits 108 and the respective slits 109. In the laminating portion, the streams of the molten resin A flowing from the outlets of the respective slits 108 and the streams of the molten resin B flowing from the outlets of the respective slits 109 form alternate layer streams of molten resins.
The respective slits 108 and 109 are formed, for example, by a comb-like rectangle (slit plate) having numerous slits formed with intervals (corresponding to the intervals 110) kept between them in the longitudinal direction (X-axis direction shown in FIG. 11) of the rectangle (or the plate) to pass through the rectangle in the width direction (Y-axis direction shown in FIG. 11) of the rectangle, extending from the bottom surface toward the top surface (Z-axis direction shown in FIG. 11) of the rectangle without reaching the top surface of the rectangle.
In the multilayer feed block 101, the molten resin A flows from the manifold 104 into the tiny holes 106 and subsequently into the slits 108. On the other hand, the molten resin B flows from the manifold 105 into the tiny holes 107 and subsequently into the slits 109.
The structure of the conventional multilayer feed block 101 explained above is also shown in the Patent Document 2. In the conventional multilayer feed block 101, the slits 108 and 109 formed in the slit plate are formed such that the slit lengths (the slit lengths in Z-axis direction shown in FIG. 11) at both the ends of each of the slits in the width direction of the slit (Y-axis direction shown in FIG. 11) are equal to each other, in view of easer machining and lower machining cost.
Therefore, when the molten resin is introduced from each of the tiny holes 106 (or 107) formed in a lateral face of the corresponding slit 108 (or 109) into the slit, as shown in FIG. 12, a length difference exists between the flow path length L1 of the resin to the outlet SO of the slit 108 (or 109) on the side near the tiny hole 106 (or 107) and the flow path length L2 of the resin to the outlet SO of the slit 108 (or 109) on the side far from the tiny hole 106 (or 107).
For this reason, the flow rate of the molten resin at the outlet SO of the slit 108 (or 109) is large at the slit outlet Son near the tiny hole 106 (or 107) and gradually decreases from there to the slit outlet Sof far from the tiny hole 106 (or 107). That is, the flow rate of the molten resin at the slit outlet Son near the tiny hole 106 (or 107) is larger than the flow rate of the molten resin at the slit outlet Sof far from the tiny hole 106 (or 107).
While the difference in the flow rate of the molten resin is kept in the width direction (Y-axis direction shown in FIG. 12) at the outlet SO of each slit, the streams of the molten resins discharged from the respective slits are laminated at the laminating portion, to form layer streams of the molten resins. The layer streams in this state are extruded from the die 5 in such a manner that the lamination direction (X-axis direction shown in FIG. 11) corresponds to the thickness direction of the produced multilayer film, in other words, in such a manner that the width direction (Y-axis direction shown in FIG. 11) of the slits corresponds to the width direction of the produced multilayer film, for forming the intended multilayer film. The thicknesses of the respective layers of the multilayer film formed like this are not constant in the width direction. That is, a multilayer film with the thicknesses of its respective layers kept uniform in the width direction cannot be obtained.
Furthermore, the conventional multilayer feed block 101 has a possibility that the molten resin is retained in the top portion of each slit far from the corresponding tiny hole 106 (or 107). If the molten resin is retained there, a problem that the resin is thermally deteriorated occurs.
Meanwhile, in FIG. 12, the series concerned with the flow of the molten resin A, including the manifold 104, the tiny hole 106 and the slit 108 and the series concerned with the flow of the molten resin B including the manifold 105, the tiny hole 107 and the slit 109 are shown in the same direction in the drawing. However, as can be seen from FIG. 11, actually one series and the other series are reverse to each other in direction.
In the multilayer feed block shown in the Patent Document 3, each slit is formed to have a circular arc at the top. This design is considered to decrease the residence of the molten resin in the top corner of each slit. However, the problem that the thicknesses of the respective layers are not uniform in the width direction of the multilayer film because of the above-mentioned difference in the flow path length of the molten resin in each slit is not solved yet.
Furthermore, since the respective slits are internally formed to have a circular arc partially, it is difficult to machine the slits especially in the case where the slit gaps are small, and furthermore since the structure needs tiny holes, there is a problem that the slit plate production cost is high. Moreover, since the top of each slit is formed to have a depressed circular arc, there is a problem that maintenance such as washing is complicated.
Known is an optical interference film capable of reflecting or transmitting light with a broad-band wavelength, in which a resin having a high refractive index and a resin having a low refractive index are alternately laminated in the thickness direction of the film at the same rates with the thickness of each pair of layers gradually decreased or increased.
If the multilayer film, in which the respective layers or respective pairs of layers change or successively change in the thickness direction of the film, is produced using the above-mentioned conventional multilayer feed block, the slit gaps of the respective slits formed in the above-mentioned slit plate must be changed in the lamination direction of the respective layers of the produced multilayer film. However, in this case, the slits must be machined at very high accuracy, and for a requested multilayer film, it becomes necessary to change sizes of gaps of the slits in the order of 1 μm or less. However, it is difficult to meet this request with the presently available machining technique alone.
The Patent Document 2 proposes to control the temperature distribution of the feed block for forming layers respectively different in thickness. However, it is difficult to accurately control the thicknesses of tens of or hundreds of layers by this method.
On the other hand, for the purpose of obtaining an optical interference film, the lamination constitution of the respective layers of a multilayer film was designed, and the above-mentioned conventional multilayer feed block was used in an attempt to mold a multilayer film. However, it was found that in the molded multilayer film, the layers closer to the surface of the film were thinner than the design layer thicknesses (intended layer thicknesses) of the film, and this phenomenon was more outstanding than the expected thickness irregularities of the respective layers. That is, it was found that it is difficult to produce a multilayer film having the intended thicknesses of respective layers using the conventional multilayer feed block.                Patent Document 1: JP 50-6860 B        Patent Document 2: JP 2003-112355 A        Patent Document 3: JP 2003-251675 A        